Victims of the Sudden Infant Death Syndrome were found, in histopathological studies, to have experienced covert hypoxia, probably a result of sleep-related respiratory depression. Our studies are concerned with factors influencing sleep-related respiratory depression as well as physiological variables predicting hypoxic responses during sleep in kittens. We found that hypoxic kittens may exhibit respiratory failure occurring exclusively in quiet sleet (QS), with normalization of breathing in active sleep (AS). Hypoxia also caused augmentation of QS and suppression of AS. Respiratory adaptations and sleep state changes in response to hypoxia in sleep were found to be influenced by ambient temperature. Elevated temperatures prevent hypoxic suppression of AS, while cooling suppresses respiratory compensation for hypoxia. Thus, temperature control may be useful in protecting high risk infants. Apnea frequency was reduced in hypoxic kittens as in certain risk groups of infants. Thus apnea may not predict respiratory failure. However, reduced amplitude in 0-7 Hz EEG activity in QS discriminated hypoxic kittens and may be useful in detecting risk infants.